Body of troops (5)
![Ross](/clue/static/ross-mug.min.png)
'body of troops' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for corps that I've seen before include "Part of an army" , "Large body of troops" , "Specialised mil. body" , "ballet group" , "Body of men" .)
(Other definitions for regiment that I've seen before include "Organise; army unit" , "Large army unit" , "Group of battalions" , "Brigade" , "subject to excessive control" .)